High-biased climate sensitivity estimates from mid-Pliocene Warm Period temperatures

The warm Pliocene epoch is used to estimate Earth's equilibrium climate sensitivity (ECS), which is the long-term temperature change after a sustained doubling of atmospheric CO₂ over pre-industrial levels. Using an emergent constraint on the relationship between mid-Pliocene Warm Period simulated temperatures and ECS, we estimate ECS to be 4.8 K, which is higher than previous studies on the Pliocene. This is partly due to using warmer SST reconstruction than before; a consequence of focusing modelling efforts on the mid-Pliocene warm period. Using the temperatures of a broader period within the Pliocene, we quantify ECS to be 3.1 K. Further uncertainties on proxy data and data-model disagreements are expected to affect ECS estimates. We find that CO₂ uncertainties are the main driver of variations in ECS estimates, followed by biases from seasonal temperatures. The bias from polar amplification is apparently small, but could be an overlooked source of error. We conclude that the Pliocene-based emergent constraint is nonetheless robust and is likely to improve further as geological reconstructions improve.